It is well-known to reinforce concrete or mortar with steel fibres to improve the quality of the concrete material.
EP-B1-851957 (NV Bekaert SA) teaches a steel fibre with flattened hook shaped ends, whereby the post-crack bending strength of the concrete, reinforced by means of such fibres, is highly improved.
U.S. Pat. No. 4,883,713 (Eurosteel) teaches a steel fibre comprising a cylindrical steel body having conically shaped ends for improving the anchoring feature of the steel fibre into the steel fibre reinforced concrete.
These two cited documents, as well as other documents, teach that the properties of conventional steel fibre concrete can be highly improved thanks to the improved anchoring features of the steel fibres into the steel fibre concrete.
Currently the known prior art steel fibres for concrete reinforcement function very well for improving the service-ability limit state (SLS) of a concrete structure, i.e. they bridge very well the cracks or crack mouth opening displacements (CMOD) up to the typically required 0.5 mm, e.g. CMOD's ranging between 0.1 mm and 0.3 mm, during a typical three point bending test—for the test see European Standard EN 14651 (Test method for metallic fibred concrete, measuring the flexural tensile strength).
In other words, known steel fibres like steel fibres with hook shaped ends and fibres having conically shaped ends function well for limiting the width or growth of cracks up to about 0.5 mm (SLS).
The disadvantage today of these fibres is the relatively low performance at ultimate limit state (ULS). Especially, the ratio between ultimate limit state (ULS) and service-ability limit state (SLS) post-crack strength is relatively low. This ratio is determined by the load value FR,1 (CMOD=0.5 mm) and FR,4 (CMOD=3.5 mm).
Some prior art fibres do not perform at ULS as they break at CMOD lower than what is required for ULS. Other fibres, like hooked end fibres are designed to be pulled-out. Due to the pull-out, those fibres show a displacement-softening behaviour already for small displacements.
In spite of this low performance at ULS, presently known steel fibres may also be used in so-called structural applications in order to improve the ultimate limit state (ULS). Here the known steel fibres are expected to bear or carry load, instead of or in addition to classical reinforcement, such as rebar, mesh, pre-stressing, and post-tensioning. In order to be effective in such load carrying function, however, these present steel fibres have to be used in huge dosages considerably exceeding normal dosages of 20 kg/m3 to 40 kg/m3. The huge dosages can cause workability problems such as the mixing and placing problems.